This factors out all of the OpenGL specific code in cogl-texture-2d.c
into cogl-texture-2d-gl.c and where necessary adds indirection through
the CoglDriver vtable so that we can eventually start to experiment with
non-OpenGL backends for Cogl.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit ec57588671696bbe7ce714bdfe7324236146c9c0)
